Question: If $5a + 8b + 9c = 10$ and $x + 2y + 3z = -2$, what is $-80b + 30z - 50a - 90c + 10x + 20y$ ?
Answer: $= -50a - 80b - 90c + 10x + 20y + 30z$ $= (-10) \cdot (5a + 8b + 9c) + (10) \cdot (x + 2y + 3z)$ $= (-10) \cdot (10) + (10) \cdot (-2)$ $= -100 - 20$ $= -120$